Mechanism of Breathing — Definition

Definition

Breathing, or pulmonary ventilation, is the physical process by which we take air into our lungs (inspiration) and expel air out of our lungs (expiration). It's a mechanical process, meaning it involves physical movement and changes in pressure, rather than chemical reactions.

Think of your lungs like balloons inside a sealed chest cavity. To inflate the balloons (inspiration), you need to make the space around them bigger, which reduces the pressure inside the chest, drawing air in.

To deflate them (expiration), you make the space smaller, increasing the pressure and pushing air out.

The primary muscles responsible for this action are the diaphragm and the intercostal muscles. The diaphragm is a large, dome-shaped muscle located at the base of the chest cavity, separating it from the abdomen.

When you breathe in, your diaphragm contracts and flattens, moving downwards. This action significantly increases the vertical volume of your thoracic cavity. Simultaneously, the external intercostal muscles, located between your ribs, contract.

This pulls your ribs upwards and outwards, increasing the volume of your thoracic cavity from front-to-back and side-to-side. The combined effect of these muscle contractions is a substantial increase in the overall volume of the thoracic cavity.

According to Boyle's Law, if the volume of a gas increases, its pressure decreases (assuming temperature is constant). So, as the thoracic cavity volume expands, the intrapulmonary pressure (pressure inside the lungs) drops below the atmospheric pressure (pressure outside your body).

This pressure difference creates a gradient, causing air to rush from the higher atmospheric pressure into the lower intrapulmonary pressure, filling your lungs. This is inspiration, an active process requiring muscle contraction.

When you breathe out, or expire, during quiet breathing, the process is largely passive. The diaphragm and external intercostal muscles simply relax. The diaphragm returns to its dome shape, moving upwards, and the ribs move downwards and inwards due to their natural elasticity and the elasticity of the lungs and thoracic wall.

This relaxation decreases the volume of the thoracic cavity. Consequently, the intrapulmonary pressure increases, becoming higher than the atmospheric pressure. This pressure gradient forces air out of the lungs until the pressures equalize.

During forced expiration, however, internal intercostal muscles and abdominal muscles actively contract to further reduce thoracic volume and expel more air. Understanding these muscle actions and the resulting pressure changes is key to grasping the mechanism of breathing.